Low loss optical waveguide device

ABSTRACT

A method for forming optical devices on planar substrates, as well as optical devices formed by the method are described. The method uses a linear injection APCVD process to form optical waveguide devices on planar substrates. The method is performed at approximately atmospheric pressure. According to the method, a wafer with a lower cladding layer already formed by either CVD or oxidation is placed on a conveyer, which may include a heating element. The heated wafer is transported underneath a linear injector such that the chemicals from the linear injector react on the wafer surface to form a core layer. After the core layer is formed, photoresist is spun on the surface of the wafer, and then standard lithography is used to pattern the optical devices. Next, reactive ion etching (RIE) is used to form waveguide lines. The remaining photoresist is then removed. An upper cladding layer is formed to substantially cover the core regions. The upper cladding layer may be formed in a manner similar to that used to form the core layer. The refractive index of the upper cladding layer is generally the same as that of the lower cladding layer. The refractive index of the core layer is generally 0.2% to 2% greater than that of the upper and lower cladding layers.

BACKGROUND OF THE INVENTION

[0001] Optical waveguide devices formed on planar substrates have becomeimportant elements for various optical network applications, includingmultiplexer and demultiplexer in dense wavelength division multiplexing(DWDM) systems and components in passive optical networks (PON). Thistechnology allows multiple functional units to be integrated on a singlesubstrate.

[0002] The key to forming optical waveguide devices on a planarsubstrate is the deposition process. In order to produce high qualitydevices, the deposition process must produce stable films thatdemonstrate low optical loss. Ideally, the deposition method shouldprovide for high throughput as well as high quality devices.

[0003] Various methods have been used to form optical waveguide deviceson a planar substrate. For example, halide materials have been used toform device layers. However, this method requires special handling ofthe corrosive halide materials. Another method that has been used isdeposition at sub-atmospheric pressure; for example, sub-atmosphericplasma-enhanced chemical vapor deposition (PECVD). However, this methodprovides a lower deposition rate than the current invention. A thirdmethod that has been used is atmospheric pressure chemical vapordeposition (APCVD) using a showerhead configuration. However, thismethod provides less than optimal wafer-to wafer uniformity than doesthe current invention.

SUMMARY OF THE INVENTION

[0004] The current invention provides a method for forming opticalwaveguide devices on a planar substrate that does not involve usingcorrosive halide materials. The method provides an improved throughputover reduced pressure methods and better wafer-to-wafer uniformity thanshowerhead APCVD methods.

[0005] The current invention uses a linear injection APCVD method toform layers for optical waveguide devices on planar substrates. A linearinjector apparatus that can be used to perform the method of the currentinvention is described in U.S. Pat. No. 5,855,957 to Yuan, which ishereby incorporated by reference.

[0006] The current invention does not use corrosive halide materials;instead it uses primarily metal-organic materials, such astetraethylorthosilicate (TEOS), trimethylphosphite (TMPi),triethylphosphate (TEPo), trimethylborate (TMB), triethylborate (TEB),and tetramethyloxygermane (TMOG).

[0007] The process is performed at approximately atmospheric pressureand therefore provides a higher deposition rate than reduced pressureprocesses. For example, a deposition rate of 0.6 μm/min has beenobtained with the process. Additionally, the linear injector methodenables the user to obtain a very uniform deposition. For example, themethod has been used to produce layers with refractive index uniformityof within ±0.0002, while layer thickness has been controlled to beuniform to within ±4%.

[0008] According to the method, a wafer with a lower cladding layeralready formed by either CVD or oxidation is placed on a conveyer; forexample, a conveyer belt transport device. The conveyer may also includea heating element to heat the wafer, although other means may be used toheat the wafer. If the wafer includes a quartz glass or fused silicasubstrate, the substrate may act as the lower cladding layer.

[0009] The linear injector transports materials to the wafer forformation of the subsequent core and upper cladding layers. Materialsused to form the core and upper cladding layers include TEOS, TMPi,TEPo, TMB, TEB, and TMOG. Oxidizing agents, for example an O₃/O₂mixture, are also used to form the core and upper cladding layers.

[0010] In order to form the core layer, TEOS is used as a source gas.The core layer may include dopants; for example, P₂O₅, GeO₂, and TiO₂may be used as dopants for the core layer. The dopants may increase therefractive index of the core layer as needed to provide the necessaryoptical properties for the resulting device. The conveyer transports theheated wafer underneath the linear injector such that the chemicals fromthe linear injector react on the wafer surface to form the core layer.

[0011] After the core layer is formed, photoresist is spun on thesurface of the wafer, and standard lithography is used to pattern theoptical devices. Next, reactive ion etching (RIE) is used to form coreregions. The remaining photoresist is then removed.

[0012] After the photoresist is removed, the upper cladding layer isformed. The upper cladding layer may be formed in a manner similar tothat used to form the core layer. The upper cladding layer may includedopants; for example, P₂O₅ or B₂O₃ may be used as dopants in the uppercladding layer. The upper cladding dopants may be used to make the glassflow better to fill in between the etched core regions. The uppercladding layer substantially covers the core regions. The refractiveindex of the upper cladding layer is generally the same as that of thelower cladding layer. The refractive index of the core layer isgenerally 0.2% to 2% greater than that of the upper and lower claddinglayers.

[0013] The current invention also includes optical waveguide devicesformed using the described method. For example, it includes ArrayWaveguide Grating (AWG) devices formed using the method.

[0014] This invention can be more fully understood in light of thefollowing detailed description taken together with the accompanyingfigures. Like elements are designated by like reference numeralsthroughout the figures.

BRIEF DESCRIPTION OF THE FIGURES

[0015]FIG. 1 is a cross sectional view of an optical waveguide device;

[0016]FIG. 2 provides a process flow diagram;

[0017]FIG. 3 is a cross sectional view of a linear injector apparatus;

[0018]FIG. 4 illustrates the method by which materials are transportedto the linear injector.

[0019]FIG. 5 is a graph of insertion loss versus wavelength for a 32channel Array Waveguide Grating (AWG) formed using the described method.

DETAILED DESCRIPTION OF THE INVENTION

[0020] An optical waveguide device formed on a planar substrate includesthree layers formed on the substrate. Referring to FIG. 1, an opticalwaveguide device 1 is formed on a planar substrate 10. The substrate 10may be the wafer itself or may be a layer formed on the surface of thewafer. Lower cladding layer 12 is doped or pure SiO₂. It may be formedby CVD or by oxidation. It may be formed using the linear injector APCVDmethod of the current invention using pure or doped TEOS. The totaldopant level for the lower cladding layer 12 is typically 0-10 wt %. Ifthe substrate 10 is quartz glass or fused silica, the substrate itselfmay act as the lower cladding layer. The thickness of lower claddinglayer 12 is generally between 2-20 μm.

[0021] Core 14 is pure or doped SiO₂ Examples of dopants include P₂O₅,GeO₂, and TiO₂. The core dopants increase the material's refractiveindex in order to obtain the required optical properties of thecompleted device. The refractive index of the core is normally 0.2% to2% greater than that of the cladding layers. The total dopant level forthe core layer is typically 1-20 wt % and the film thickness istypically 1-10 μm.

[0022] The upper cladding layer 16 is pure or doped SiO₂. For example,P₂O₅ or B₂O₃ may be used as dopants in the upper cladding layer. Therefractive index of the upper cladding layer 16 is generally matched tothe refractive index of the lower cladding layer 12. The total dopantlevel for the upper cladding layer 16 is generally 0-15 wt % and thethickness is typically 2-20 μm.

[0023]FIG. 2 illustrates the process of the current invention. In step100, the lower cladding layer 12 has already been formed by CVD oroxidation on substrate 10. The core layer 14A is formed on top of thelower cladding layer using the method of the invention. In step 200,photoresist layer 15 is spun on to the surface of core layer 14A. Instep 300, the waveguide patterns are defined using standard lithographytechniques. In step 400, cores 14 are formed with reactive ion etching(RIE) using standard etching techniques. In step 500, the residualphotoresist material is removed. In step 600, the upper cladding layer16 is formed using the method of the invention. The upper cladding layersubstantially covers the core structure.

[0024]FIG. 3 shows an apparatus that may be used to perform the methodof the current invention. Wafer 20 is moved through reaction chamber 28by conveyer 22. The conveyer 22 may include a heating element whichheats the wafer (not shown). For example, the wafer may be heated toapproximately 500° C. Alternately, other methods may be used to heat thewafer. The wafer is heated in order to allow the raw materials to reacton the surface of the wafer to form the necessary layers.

[0025] Raw material source lines 26 (which may transport TEOS, dopantsource materials such as TMOG, TMPi, MEPo, TMB, and TEB, or oxidizingagents) transport the raw materials to the linear injector 24. Theoxidizing agent used in the process is typically an O₃/O₂ mixture; forexample, 30 g/m³ of O₃ in O₂. The raw materials are transported throughone or more injection ports 17 toward heated wafer 20. When thematerials reach the heated wafer, they react with the surface materialand form a layer on the surface. The linear injector does not provideraw materials to the entire surface of the wafer at one time; insteadthe raw materials are provided over an exposure area that depends on thegeometry of the injector and the distance between the injector ports andthe wafer. The entire surface of the wafer passes through the exposurearea as the wafer 20 is moved through the reaction chamber 28 on theconveyer 22.

[0026] By-products of the reaction and unreacted gases may be removedfrom the chamber 28 through exhaust ports 18 positioned on either sideof the injector 24.

[0027]FIG. 4 illustrates the method by which raw materials such as TEOSmay be transported to the linear injector. Carrier gas 30, for examplenitrogen, enters the bubbler 32 which contains the desired material; forexample, TEOS. As the carrier gas passes through the material it createsbubbles containing the vapor of the material. A mixture 34 of carriergas and vapor of one or more desired materials flows into the linearinjector and then mixes with oxidizing agents. By adjusting andcontrolling the carrier gas flow rates to the bubblers, the amount ofvapor for each material can be precisely maintained. The followingformula is used to calculate the amount of each material fed into thelinear injector:

n=[P _(v)/(760−P _(v))]×(f/22.4)

[0028] Where n=the number of moles per minute of the material fed intothe linear injector, P_(v)=vapor pressure of the material in torr, andf=the carrier gas flow rate in standard liters per minute.

[0029] Additional bubblers 35 may be used to provide additionalmaterials, for example they may hold the source materials for one ormore dopants.

EXAMPLE

[0030] The method of the invention was performed in the followingmanner. A silicon wafer with an oxide lower cladding layer was provided.A core layer of SiO₂ doped with 8 wt % P₂O₅ was then formed using themethod described above. The waveguide structure was patterned usingstandard photolithography and RIE techniques. Then an upper claddinglayer covering the core structure was formed using SiO₂ doped with 2 wt% P₂O₅ and 5 wt % B₂O₃.

[0031] The loss values for waveguides produced using this method were0.1 dB/cm for straight waveguides and 0.25 dB/cm for curved waveguides.

[0032] Additionally, array waveguide grating devices for wavelengthdivision multiplexing and demultiplexing applications have beenfabricated using this method. They exhibited less than 6 dB loss andapproximately 30 dB cross talk. FIG. 6 shows a graph of insertion lossversus wavelength for a 32 channel Array Waveguide Grating (AWG) formedusing the described method.

[0033] The preceding example illustrates one embodiment of theinvention. Other embodiments of the invention can be used as well. Forexample, good results were obtained using SiO₂ doped with 7-9 wt % P₂O₅for the core layer and SiO₂ doped with 1-2 wt % P₂O₅ and 3-5wt % B₂O₃for the upper cladding layer.

[0034] The above-described embodiments of the present invention aremerely meant to be illustrative and not limiting. It will thus beobvious to those skilled in the art that various changes andmodifications may be made without departing from this invention in itsbroader aspects.

We claim:
 1. A method for producing an optical waveguide device,comprising: providing a wafer comprising a wafer material positioned ona conveyer, said wafer providing a lower cladding material; providing alinear injector positioned to transport a layer formation material ontoan exposure area, such that when said wafer is conveyed through saidexposure area said layer formation material can form a layer on saidwafer; forming a core layer on said lower cladding material by conveyingsaid wafer on said conveyer through said exposure area while said linearinjector transports a core layer formation material onto said exposurearea at approximately atmospheric pressure; forming a core by etchingsaid core layer; and forming an upper cladding layer on said core regionby conveying said wafer on said conveyer through said exposure areawhile said linear injector transports an upper cladding layer formationmaterial onto said exposure area at approximately atmospheric pressure,such that said upper cladding layer substantially covers said core. 2.The method of claim 1 wherein the lower cladding material is a lowercladding layer on said wafer.
 3. The method of claim 1 wherein the lowercladding material is the wafer material.
 4. The method of claim 1,further comprising the step of heating the wafer.
 5. The method of claim1, wherein the core layer formation material includes TEOS.
 6. Themethod of claim 1, wherein the core layer formation material includesTEPo.
 7. The method of claim 1, wherein the core layer formationmaterial includes TMPi.
 8. The method of claim 1, wherein the core layerformation material includes TMOG.
 9. The method of claim 1, wherein thecore layer formation material includes an oxidizing agent.
 10. Themethod of claim 1, wherein the upper cladding layer formation materialincludes TEOS.
 11. The method of claim 1, wherein the upper claddinglayer formation material includes TMPi.
 12. The method of claim 1,wherein the upper cladding layer formation material includes TEPo. 13.The method of claim 1, wherein the upper cladding layer formationmaterial includes TEB.
 14. The method of claim 1, wherein the uppercladding layer formation material includes TMB.
 15. The method of claim1, wherein the upper cladding layer formation material includes anoxidizing agent.
 16. An optical waveguide device made according to theprocess of claim 1.